MC908JB12DWE Datasheet, PDF (136/332 Pages) Freescale Semiconductor, Inc – MC68HC908JB16 Technical Data

Monitor ROM (MON)
Upon power-on reset, if the received bytes of the security code do not
match the data at locations $FFF6–$FFFD, the host fails to bypass the
security feature. The MCU remains in monitor mode, but reading a
FLASH location returns an invalid value and trying to execute code from
FLASH causes an illegal address reset. After receiving the eight security
bytes from the host, the MCU transmits a break character, signifying that
it is ready to receive a command.
NOTE: The MCU does not transmit a break character until after the host sends
the eight security bytes.
To determine whether the security code entered is correct, check to see
if bit 6 of RAM address $80 is set. If it is, then the correct security code
has been entered and FLASH can be accessed.
If the security sequence fails, the device should be reset by a power-on
reset and brought up in monitor mode to attempt another entry. After
failing the security sequence, the FLASH module can also be mass
erased by executing an erase routine that was downloaded into internal
RAM. The mass erase operation clears the security code locations so
that all eight security bytes become $FF (blank).
9.5.1 Extended Security
To further disable monitor mode functions, the monitor commands can
be disabled by writing $7B to the FLASH location $FFD1 and $87 to the
FLASH location $FFD0. Table 9-10 shows the security settings that
affect monitor mode operations.
Extended Security
NOT SET
SET
Table 9-10. Monitor Mode Security
Monitor Mode Entry Security
BYPASSED
FAILED
BYPASSED
FAILED
Monitor Functions Available
Read/write of RAM and FLASH.
Read/write of RAM.
Read of FLASH disabled. FLASH can only
be mass erased.
Read/write of RAM and FLASH disabled.
Read/write of RAM.
Read of FLASH disabled. FLASH can only
be mass erased.
Technical Data
136
Monitor ROM (MON)
MC68HC908JB16 — Rev. 1.1
Freescale Semiconductor